(1) Field of the Invention
The invention relates generally to electronic oscillators and in particular to improved crystal resonator, thin-film resonator or micro electromechanical resonator oscillators, realized with monolithic integrated-circuit technologies, where one chip solutions include automatic amplitude control and biasing to accommodate a highly accurate frequency-generation exhibiting low phase noise and stable amplitudes at higher frequencies.
(2) Description of the Prior Art
Most crystal oscillators in monolithic integrated circuit technology are developed using Pierce oscillator circuit schemes, where the frequency determining resonator is working in parallel resonance mode. Realized with quartz crystals as resonators normally only narrow band tuning is featured and phase noise is considered sufficiently good at frequency offsets not too far away from the oscillator carrier signal. It would be advantageous to extend the tuning range whilst maintaining a good phase noise behavior at far away offsets.
Crystal-controlled oscillators have been in use for decades in electronic systems as frequency references; but such oscillators have mostly been implemented using bi-polar transistors as active elements. However, the dominant technology for the fabrication of most integrated circuits today is CMOS and design techniques for highly stable crystal oscillators in this technology are less well known, especially when it comes to frequencies of about 100 MHz, as necessary for modem communication applications.
In the prior art, there are different technical approaches for achieving the goals of good tuneability and low phase noise. These crystal oscillator arrangements always include a piezo-electric, e.g. quartz, crystal and drive current means therefore. Unfortunately, these approaches are somewhat expensive, both in terms of technical complexity (e.g. differential push-pull or balanced bridge structures, extra filter or tank circuits, sophisticated temperature compensation or gain control circuits, amplitude peak detectors etc.) andxe2x80x94hence xe2x80x94commercial costs. It would be advantageous to reduce both expenses. This is achieved by using an oscillator circuit working with a crystal in parallel resonance mode, originating from Pierce. Using the intrinsic advantages of that solutionxe2x80x94as described later on in every detailxe2x80x94the circuit of the invention is realized with standard CMOS technology at low cost.
Several prior art inventions describe related crystal oscillators.
U.S. Pat. No. (5,528,201 to Davis) describes a Pierce crystal oscillator having reliable start-up for a digital integrated circuit implementation, which has a capacitance element (such as a field effect capacitor) of an appropriate capacitance value disposed on-board the integrated circuit. One lead of the capacitance element is coupled to the input lead of the gain stage of the Pierce oscillator circuit whereas a second lead of the capacitance element is coupled to the output lead of the gain stage. Providing the capacitance element facilitates oscillator start-up and reliability by effectively eliminating the upper gain limit for oscillation. Specific circuit embodiments are also disclosed.
U.S. Pat. No. (6,052,036 to Enstrom et al.) discloses a highly stable single chip crystal controlled oscillator with automatic gain control and on-chip tuning. An amplitude detector monitors the output of a crystal controlled oscillator amplifier and produces a feedback signal proportional to the output signal of the amplifier to ensure oscillation is induced at start-up and that the amplitude of oscillation is limited to a preselected value during operation to conserve power consumption by the amplifier. The capacitor tank circuit connected to the input of the amplifier includes a voltage variable capacitor the voltage across which is initially established at manufacture to tune the oscillation frequency to a preselected value. The voltage across the voltage variable capacitor is also adjusted to compensate for temperature variations in the circuit.
U.S. Pat. No. 6,194,973 to Williamson) shows an oscillator with automatic gain control, where an oscillator having an adjustable gain circuit provides abundant gain when the oscillator is first powered up but reduces the gain substantially below its start-up value once oscillations build up, thereby substantially reducing the power consumed. The oscillator comprises an inverting amplifier coupled to a resonator, an oscillation detector coupled to the inverting amplifier, and a common-gate amplifier coupled to the oscillation detector. The inverting amplifier amplifies oscillations of the resonator according to a gain. The oscillation detector outputs a detection signal in response to oscillations of the resonator. The level of the detection signal is proportional to the amplitude of the oscillations. The common-gate amplifier receives the detection signal and, in response, limits the current to the inverting amplifier to control the gain based on the level of the detection signal.
U.S. Pat. No. (6,278,338 to Jansson) discloses a crystal oscillator with peak detector amplitude control. A crystal oscillator apparatus is described that has a wide dynamic frequency range and that is capable of supporting a broad range of crystal types. The present invention reduces the unwanted side effects that are associated with the prior art crystal oscillator designs, such as the clipping of signals, the introduction of signal distortion and unwanted signal harmonics. The present invention reduces the total wasted loop gain of the oscillator while also reducing the amount of integrated circuit real estate required to implement the crystal oscillator. The crystal oscillator apparatus of the present invention preferably comprises a crystal resonator circuit, an inverting amplifier, a bias circuit, a reference circuit, and a peak detector circuit. The present invention takes advantage of automatic gain control design techniques. The gain of the present crystal oscillator is automatically regulated using a closed loop circuit design. The present invention advantageously utilizes a peak detector circuit in combination with a reference circuit. The peak detector compares a reference signal with an amplified and inverted oscillation signal produced by a crystal resonator, and generates a feedback signal as a result of the comparison. The feedback signal controls a bias circuit that, in turn, controls the amplified inverted oscillation signal.
A principal object of the present invention is to provide an effective and very manufacturable method and circuit for generating resonator stabilized oscillation signals. The results are especially applicable and very efficient for use with resonators at least made up of Quartz or Piezo crystals, or of Thin-Film Resonators (TFR) or Micro Electro Mechanical System (MEMS) resonators but not restricted to only those types of resonators.
Another further object of the present invention is to attain low amplitude distortion of the oscillator signal.
Another still further object of the present invention is to reach a low phase noise behaviour of the circuit, i.e. to attain a high frequency stability.
Also an object of this invention is the starting up of the generation process of the oscillation signal at maximum speed.
Another object of this invention is minimizing the distortion and the phase noise of the generated oscillation signal in steady state operation.
A still further object of the present invention is to reduce the power consumption of the oscillator circuit by realizing inherent appropriate design features.
Another further object of the present invention is to reduce the cost of manufacturing by implementing the circuit as a monolithic integrated circuit in low cost CMOS technology.
Another still further object of the present invention is to reduce cost by minimizing the chip area by means of effectively minimizing component values.
In accordance with the objects of this invention, a circuit, capable of generating a crystal controlled oscillator output signal is achieved. Said circuit comprises means for generating an oscillating signal having a stable and predetermined oscillation frequency with means for driving said oscillation generating means and for matching the impedance levels between said oscillation generating means and this oscillator driving means. Also comprised are means for automatic amplitude controlling and biasing the generated oscillation signal of said oscillation generating means and said oscillator driving means, thus controlling the amplification factor (gain) of said oscillator driving means in such a way, that a steady oscillation signal is maintained. Thereby biasing the circuits of said oscillation generating means and said oscillator driving means in such a way, that an optimized operating mode is obtained. The circuit further comprises means for a first buffering of said generated oscillation signal of said oscillation generating means and said oscillator driving means as a sine wave signal, named also as analog signal. Equally comprised are means for a second buffering of said generated oscillation signal of said oscillation generating means and said oscillator driving means as a digitally transformed, rectangular square wave signal. Still further comprised are means for generating the start-up control signals for said oscillation generating means and said driving oscillator means to ensure a safe start-up procedure. Finally comprised are means for generating the power-up and shut-down control signals for said oscillator driving means, for said automatic amplitude controlling and biasing means, for said first buffering means and said second buffering means, and for said start-up means.
Also in accordance with the objects of this invention, a method for generating a stable, amplitude controlled oscillation signal within an electronic device or technology is given. Said method includes providing a resonator element for determination of the oscillator frequency and providing a Pierce oscillator circuit for driving this resonator element. The method further comprises providing an automatic amplitude control circuit for stabilizing the oscillation signal together with providing a biasing circuit for the amplification of the oscillation signal. Also included in the method is providing an analog output buffer circuit for isolating the oscillator circuit from load influences and providing a digital output buffer circuit for transforming the sine wave oscillator circuit into a square wave signal. Finally comprised is providing a start-up circuit for establishing secure start-up conditions for the Pierce oscillator and the automatic amplitude controller and biasing circuit just like providing a power-up circuit for fast power-up action and for enabling an operation in power saving mode.